On Thu, Jan 7, 2021 at 10:01 AM Axel Wagner
<axel.wagner...@googlemail.com> wrote:
>
> Why do you need fields? If you need fields, you are likely putting too many
> things into your context (note, that the lookup is O(N), with N being the
> number of distinct keys - so you want the number of distinct keys to be
> small). The intended use of `context.WithValue` is to only have one key per
> "use-case" (normally, that means one key per package) and then store the
> actual data in the value.
>
> So, yes, you *can* have fields and if you have fields and re-use one type for
> many keys that way. And if you do that, you need to specify the exact field
> values on the lookup as well. But if you want "many keys" from a single type,
> that's a code-smell and I would suggest to change your design. If you think
> you need it, feel free to expand on your use-case and we might come up with a
> better solution :)
This is what I have now in a HTTP handler function:
Attach the data:
c := requestContext{
requestID: "request-123-abc",
clientID: "client-451",
}
currentCtx := req.Context()
newCtx := context.WithValue(currentCtx, requestContext{}, c)
req = req.WithContext(newCtx)
Retrieve the data somewhere else during processing the request:
ctx := req.Context()
v := ctx.Value(requestContext{})
if m, ok := v.(requestContext); ok {
log.Printf("Processing request: %v", m)
}
Is this inefficient?
>
>
> On Wed, Jan 6, 2021 at 11:52 PM Amit Saha <amitsaha...@gmail.com> wrote:
>>
>> On Thu, Jan 7, 2021 at 9:48 AM Axel Wagner
>> <axel.wagner...@googlemail.com> wrote:
>> >
>> >
>> >
>> > On Wed, Jan 6, 2021 at 11:39 PM Amit Saha <amitsaha...@gmail.com> wrote:
>> >>
>> >> Is it fair to say then that any user-defined type here is acceptable as a
>> >> key?
>> >
>> >
>> > No. You can create a user-defined type out of *any* type T, by just
>> > writing `type X T`. In particular, you can write `type X func()` and get
>> > an incomparable user-defined type.
>> >
>> > I highly recommend sticking to the recommendation from the docs: Use `type
>> > ctxKey struct{}` as a key type and `ctxKey{}` as the key - it's comparable
>> > and the only value equal to `ctxKey{}` is `ctxKey{}`, which gives exactly
>> > the desirable semantics. And as it's zero-sized, it also doesn't occur any
>> > allocation (at least with gc, all zero-sized objects have the same
>> > address). You could also use `[0]int` or similar, but `struct{}` is very
>> > common for use-cases like this.
>>
>> Thanks. If I wanted to have a struct like this
>>
>> type myReqContext struct {
>> ID1 string
>> ID2 string
>> }
>>
>> Would the correct key and value be: myReqContext{}, myReqContext{ID1:
>> "abcd", ID2: "defg"}?
>>
>> The key isn't zero value any more, or is it? If so, should I have a
>> dedicated empty struct for the key?
>>
>>
>>
>> >
>> >>
>> >>
>> >>
>> >> >>
>> >> >> Thanks,
>> >> >> Amit.
>> >> >>
>> >> >> --
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